Attachment of different structures is crucial in a wide variety of processes. However, this is frequently associated with problems of different nature depending on what structures are to be attached.
Areas that are particularly troublesome are adhesion in the medical field, and attachment of components of very small size, such as in the micro- and nano-techniques. In the medical field, examples of when adhesives have to be used to adhere biological material include repair of lacerated or otherwise damaged organs, especially broken bones and detached retinas and corneas. Dental procedures also often require adhesion of parts to each other, such as during repair of caries, permanent sealants and periodontal surgery. It is very important in biomedical applications of an adhesive and coating composition to use bio-acceptable and biodegradable components, which furthermore should not per se or due to contamination induce any inflammation or toxic reactions. In addition, the adhesive has to be able to attach structures to each other in a wet environment. In the electronic industry, a particular problem today is that the components that are to be attached to each other often are of very small size, and the amount of adhesive that is possible to use is very small. Adhesives that provide high adhesive strength even with minor amounts of adhesive are therefore required. Also for non-medical uses, an adhesive that is non-irritating, non-allergenic, non-toxic and environmentally friendly is preferred, in contrast to what many of the adhesives commonly used today usually are.
Polyphenolic proteins, preferentially isolated from mussels, are known to act as adhesives. Examples of such proteins can be found in e.g. U.S. Pat. No. 4,585,585. Their wide use as adhesives has been hampered by problems related to the purification and characterisation of the adhesive proteins in sufficient amounts. Also, mostly when using the polyphenolic proteins as adhesives the pH has had to be raised to neutral or slightly basic (commonly to from 5.5 to 7.5) in order to facilitate oxidation and curing of the protein. However, this curing is slow and results in poor adhesive strength and therefore oxidisers, fillers and cross-linking agents are commonly added to decrease the curing time and obtain a stronger adhesive.
Mussel adhesive protein (MAP) is formed in a gland in the foot of byssus-forming mussels, such as the common blue mussel (Mytilus edulis). The molecular weight of MAP from Mytilis edulis is about 130.000 Dalton and it has been disclosed to consist of 75–80 closely related repeated peptide sequences. The protein is further characterised by its many epidermal growth factor like repeats. It has an unusual high proportion of hydroxy-containing amino acids such as hydroxyproline, serine, threonine, tyrosin, and the uncommon amino acid 3,4-dihydroxy-L-phenylalanine (Dopa) as well as lysine. It may be isolated either from natural sources or produced biotechnologically. U.S. Pat. No. 5,015,677 as well as U.S. Pat. No. 4,585,585 disclose that MAP has very strong adhesive properties after oxidation and polymerisation, e.g. by the activity of the enzyme tyrosinase, or after treatment with bifunctional reagents.
MAP is previously known to be useful as an adhesive composition e.g. for ophthalmic purposes. Robin et al., Refractive and Corneal Surgery, vol. 5, p. 302–306, and Robin et al., Arch. Ophthalmol., vol. 106, p. 973–977, both disclose MAP-based adhesives comprising an enzyme polymiser. U.S. Pat. No. 5,015,677 also describes a MAP-based adhesive containing a cross-linking agent and optionally a filler substance and a surfactant. Preferred cross-linking agents according to U.S. Pat. No. 5,015,677 are enzymatic oxidising agents, such as catechol oxidase and tyrosinase, but sometimes also chemical cross-linking agents, such as glutaraldehyde and formaldehyde can be used. Examples of fillers are proteins, such as casein, collagen and albumin, and polymers comprising carbohydrate moieties, such as chitosan and hyaluronan. U.S. Pat. No. 5,030,230 also relates to a bioadhesive comprising MAP, mushroom tyrosinase (cross-linker), SDS (sodium dodecyl sulfate, a surfactant) and collagen (filler). The bioadhesive is used to adhere a cornea prosthesis to the eye wall.
EP-A-343 424 describes the use of a mussel adhesive protein to adhere a tissue, cell or another nucleic acid containing sample to a substrate during nucleic acid hybridisation conditions, wherein the mussel adhesive protein, despite the harsh conditions encountered during the hybridisation, provided adherence. U.S. Pat. No. 5,817,470 describes the use of mussel adhesive protein to immobilise a ligand to a solid support for enzyme-linked immunoassay. Mussel adhesive protein has also been used in cosmetic compositions to enhance adherence to nails and skin (WO 88/05654).
A major problem associated with known MAP-based bioadhesive compositions, despite the superior properties of MAP per se, is that some constituents, in particular the presently used cross-linking agents, can harm and/or irritate living tissue and cause toxic and immunological reactions. Chemical crosslinking agents, such as glutaraldehyde and formaldehyde, are generally toxic to humans and animals, and it is highly inappropriate to add such agents to a sensitive tissue, such as the eye. Enzymes, such as catechol oxidase and tyrosinase, are proteins, and proteins are generally recognised as potential allergens, especially in case they originate from a species other than the patient. Because of their oxidising and hydrolysing abilities, they can also harm sensitive tissue.
Therefore, there is still a need for new adhesive compositions, both for medical and other applications, that provide strong adhesion with small amounts of adhesive, that are simple to use and that do not cause toxic and allergic reactions.